With the evolution of market trends, display screens of handheld electronic devices such as smart phones and tablets have been improved toward high resolution and large dimension for consumers to get better viewing and handling for enjoying films or mobile games. The increased dimensioned body also results in inconvenience of carrying. Therefore, flexible display screens have gradually attracted the attention of mobile phone manufacturers. From home TV, computer display screens to smart phones, tablets and other applications, foldable smart phones, tablets, etc. have been developed to achieve the effect of large-size displaying and easy carrying. In order to withstand repeated display folding and unfolding, it is desirable to develop a foldable display device with hinge mechanisms which can support a flexible display screen and allow smooth and stable folding and unfolding of the same.